Method and apparatus to limit maximum switch current in a switching power supply

ABSTRACT

Techniques are disclosed to limit the current in a switch of a switching power supply. An example switching regulator circuit includes a power switch to be coupled to an energy transfer element of a power supply. A controller to generate a drive signal is coupled to be received by the power switch to control the switching of the power switch. A short on time detector is included in the controller. The short on time detector is to detect an occurrence of a threshold number of one or more consecutive short on times of the switch. A frequency adjuster is also included in the controller and coupled to the short on time detector. The frequency adjuster is to adjust an oscillating frequency of an oscillator included in the controller in response to the short on time detector.

REFERENCE TO PRIOR APPLICATION

This application is a divisional of U.S. application Ser. No. 11/177,091 file Jul. 8, 2005, now pending, entitled “METHOD AND APPARATUS TO LIMIT MAXIMUM SWITCH CURRENT IN A SWITCHING POWER SUPPLY.”

BACKGROUND

1. Technical Field

The present invention relates generally to electronic circuits, and more specifically, the invention relates to switched mode power supplies.

2. Background Information

Switches in switching power supplies can sustain damage from excess voltage, excess current, or particular combinations of voltage and current. The instantaneous voltage and current must remain within a boundary defined as the safe operating area to prevent damage to the switch. Therefore, controllers for switching power supplies usually measure voltage and current for the purpose of protecting the switch as well as for regulating an output.

Conflicting requirements and limitations of real devices often make it impossible or impractical for controllers to measure the quantities necessary to protect the switch under all conditions. Whereas maximum voltage on the switch can usually be deduced from a simple measurement of the dc input voltage, measurement of the current in the switch is usually much more difficult.

Controllers typically must mask the measurement of switch current at certain times in the switching period to avoid false indications of excess current. Moreover, there will always be some delay between the detection of excess current and an appropriate response. Thus, conventional methods may be unable to protect the switch from damage under certain conditions of transient loading or faults.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed illustrated by way of example and not limitation in the accompanying Figures.

FIG. 1 is a functional block diagram of one embodiment of a switching power supply that may limit peak current in a power switch in accordance with the teaching of the present invention.

FIG. 2 is a section of a functional block diagram of one embodiment of a switching power supply that shows the contribution of parasitic capacitance to the current in the power switch.

FIG. 3 shows voltage and current waveforms from one embodiment of a switching power supply that may limit peak current in a power switch in accordance with the teaching of the present invention.

FIG. 4 illustrates important parameters of current waveforms from a switching power supply that does not limit peak current in a power switch in accordance with the teaching of the present invention.

FIG. 5 is a flow diagram that illustrates a method to limit peak current in a power switch in accordance with the teaching of the present invention.

FIG. 6 is a functional block diagram of an integrated circuit that limits peak current in an included power switch in accordance with the teachings of the present invention.

DETAILED DESCRIPTION

Embodiments of a power supply regulator that may be utilized in a power supply are disclosed. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. Well-known methods related to the implementation have not been described in detail in order to avoid obscuring the present invention.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “for one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

Techniques are disclosed to prevent excess current in a switch of a switching power supply. For example, a method and apparatus are disclosed to prevent damage to a power transistor in a switching power supply from conditions that are outside its safe operating area. During start-up or overload when the power supply has a low output voltage, traditional techniques that respond to excessive current in the power switch cannot prevent the current from going higher in each switching period because leading edge blanking time and current limit delay time impose a minimum on time of the switch while the low output voltage causes the controller to demand a minimum off time of the switch. To overcome the problems with the traditional techniques of direct measurement of current in the switch, a disclosed method uses a measurement of time as an indirect indication of excess current in the switch. For instance, one disclosed technique measures the conduction time of the switch rather than the current in the switch to recognize a condition of uncontrolled increasing current. A short conduction time implies excessive switch current. The response to one or more short conduction times is a delay in the next switching period, effectively extending the off time to allow the current to decrease to a safe value before the next on time of the switch.

To illustrate, FIG. 1 shows generally a functional block diagram of a power supply for an embodiment of a power supply that limits peak switch current in accordance with the teachings of the present invention. The topology of the example power supply illustrated in FIG. 1 is a flyback regulator. It is appreciated that there are many topologies and configurations of switching regulators, and that the flyback topology shown in FIG. 1 is provided for explanation purposes and that other types of topologies may also be employed in accordance with the teachings of the present invention.

As illustrated in the power supply example of FIG. 1, an energy transfer element T1 125 is coupled between an unregulated input voltage V_(IN) 105 and a load 165 at an output of the power supply. A power switch S1 120 is coupled to the primary winding 175 at an input of energy transfer element 125 to regulate the transfer of energy from the unregulated input voltage V_(IN) 105 to the load 165 at the output of the power supply. A controller 145 is coupled to generate a drive signal 156 that is coupled to be received by the power switch S1 120 to control switching of power switch S1 120. As will be discussed below, controller 145 also includes a short on time detector to sense power switch S1 to detect an occurrence of a threshold number of one or more consecutive short on times of power switch S1 120. For one embodiment, a frequency adjuster is also included in the controller 145 and coupled to the short on time detector. The frequency adjuster is used for one embodiment to adjust an oscillating frequency of an oscillator included in the controller in response to the short on time detector.

In the example of FIG. 1, the energy transfer element T1 125 is illustrated as a transformer with two windings. A primary winding 175 has N_(P) turns with an inductance L_(P). A secondary winding has N_(S) turns. In general, the transformer can have more than two windings, with additional windings to provide power to additional loads, or to provide bias voltages, or to sense the voltage at a load, or the like.

A clamp circuit 110 is coupled to the primary winding 175 of the energy transfer element T1 125 to control the maximum voltage on the power switch S1 120. As mentioned, power switch S1 120 is switched on and off with a drive signal 156 generated by a controller circuit 145. For one embodiment, switch S1 120 is a transistor such as for example a power metal oxide semiconductor field effect transistor (MOSFET). For one embodiment, controller 145 includes integrated circuits and discrete electrical components. The operation of switch S1 120 produces pulsating current in the rectifier D1 130 that is filtered by capacitor C1 135 to produce a substantially constant output voltage V_(O) or a substantially constant output current I_(O) at the load 165.

The output quantity to be regulated is U_(O) 150, that in general could be an output voltage V_(O), an output current I_(O), or a combination of the two. A feedback circuit 160 is coupled to the output quantity U_(O) 150 to produce a feedback signal U_(FB) 155 that is an input to the controller 145. Another input to the controller 145 is the current sense signal 140 that senses a current I_(D) 115 in switch S1 120. Any of the many known ways to measure a switched current, such as for example a current transformer, or for example the voltage across a discrete resistor, or for example the voltage across a transistor when the transistor is conducting, may be used to measure current I_(D) 115. The controller may use current sense signal 140 to regulate the output U_(O) 150 or to prevent damage to the switch S1 120.

FIG. 1 also illustrates an example waveform for current I_(D) 115 through power switch S1 120 under ideal conditions. The power switch S1 120 conducts for a time t_(ON) in response to a pulse in the drive signal 156 from controller 145. Power switch S1 120 is open for an off time t_(OFF) in response to the drive signal 156 from controller 145. During the time of conduction t_(ON), the current increases linearly with time from an initial current I_(VAL) to a final current I_(PEAK) in response to the input voltage V_(IN) 105 that is imposed across the inductance L_(P) of the primary winding 175 of the transformer T1 125 when the switch S1 120 is conducting.

For one embodiment, controller 145 operates switch S1 120 to substantially regulate the output U_(O) 150 to its desired value. For one embodiment, controller 145 increases conduction time t_(ON) of the switch S1 120 when output U_(O) 150 is less than its desired value. For one embodiment, controller 145 decreases conduction time t_(ON) of the switch S1 120 when output U_(O) 150 is greater than its desired value. For one embodiment, controller 145 maintains a constant conduction time t_(ON) of the switch S1 120 when switch S1 120 conducts, and the off time t_(OFF) is adjusted to regulate the output U_(O) 150. For one embodiment, switching periods are skipped to make discrete adjustments to the off time t_(OFF).

For one embodiment, controller 145 adjusts the operation of the switch S1 120 with the drive signal 156 to prevent operation outside its safe operating area. For one embodiment, controller 145 reduces conduction time t_(ON) of the switch S1 120 when current I_(D) 115 exceeds a current limit I_(LIMIT). For one embodiment, controller 145 increases off time t_(OFF) when current I_(D) 115 exceeds a current limit I_(LIMIT).

FIG. 2 shows the contribution of stray capacitance to the current I_(D) 215 that is measured in switch S1 220. Charging and discharging of stray capacitance represented by capacitors C_(P) 210 and C_(DS) 230 coupled to switch S1 at node 235 augments the current I_(D) 215 for a short time after switch S1 220 turns on. FIG. 3 shows the voltage V_(P) 270 on inductance L_(P) of primary winding 275 of transformer T1 225. For one embodiment, voltage V_(P) is positive with magnitude V_(IN) during the on time t_(ON). For one embodiment, voltage V_(P) is negative with magnitude V_(OR) during the off time t_(OFF). The abrupt reversal of polarity in voltage V_(P) adds a leading edge current to the initial current I_(VAL) of switch S1 220. The waveforms in FIG. 2 and FIG. 3 show a peak leading edge current I_(LEPEAK) that is greater than the ideal initial current I_(VAL). For one embodiment, the peak leading edge current I_(LEPEAK) is also greater than final current I_(PEAK). The stray capacitance that is present in every practical circuit can produce a high leading edge current that can interfere with the ability of a controller to regulate an output or to protect a switch. A controller that responds to the magnitude of switch current I_(D) 215 to regulate an output or to protect a switch would also respond to the leading edge current that is not significantly related to the regulation of the output or to the safe operation of the switch. To avoid interference from leading edge current, controllers typically mask the measurement of current in the switch until after a leading edge blanking time t_(LEB). The relationship of leading edge blanking time t_(LEB) to the conduction time t_(ON) is shown in FIG. 3.

The leading edge blanking time t_(LEB) is long enough to guarantee that the contribution of current from stray capacitance is negligible before the current I_(D) becomes visible to the controller. FIG. 3 also shows current limit delay time t_(d) that is the difference between the time I_(D) reaches I_(LIMIT) after t_(LEB) and the time when the switch stops conducting. Current limit delay time t_(d) is always present because practical circuits cannot respond instantaneously. Finite current limit delay time t_(d) has a consequence of peak current I_(PEAK) at the end of conduction time t_(ON) being greater than current limit I_(LIMIT).

Leading edge blanking time t_(LEB) and current limit delay t_(d) can make it impossible for typical controllers to limit the switch current I_(D). FIG. 4 illustrates an undesirable situation that can occur in a switching power supply with a typical controller when the output is less than its regulated value. In FIG. 4, an overload at the output causes the switch current I_(D) to exceed I_(LIMIT) in every switching period. The overload has also caused the output voltage V_(O) to drop far below its regulated value. The controller measures switch current I_(D) at the end of leading edge blanking time t_(LEB), and the switch opens after current limit delay time t_(d). The detection of I_(D) greater than I_(LIMIT) cannot reduce the on time t_(ON) below the sum of t_(LEB) plus t_(d). For one embodiment of the present invention, a short on time may be considered as the on time of the switch, or for example a pulse width of the drive signal, having an on time approximately equal to the sum of t_(LEB) plus t_(d).

FIG. 4 shows that in any switching period (n), the current that appears as switch current I_(D) during the on time (exclusive of any current from parasitic capacitance) increases by amount ΔI_(ON) during the on time to reach I_(PEAK), and then decreases from I_(PEAK) by an amount ΔI_(OFF) during the off time. The changes in current are related to the input and output voltages by Equation 1 and Equation 2.

$\begin{matrix} {{\Delta\; I_{ON}} = {\frac{V_{IN}t_{ON}}{L_{P}} = \frac{V_{IN}\left( {t_{LEB} + t_{d}} \right)}{L_{P}}}} & {{Equation}\mspace{14mu} 1} \\ {{\Delta\; I_{OFF}} = {\frac{V_{OR}t_{OFF}}{L_{P}} = {\left( \frac{N_{P}}{N_{S}} \right)\frac{V_{O}t_{OFF}}{L_{P}}}}} & {{Equation}\mspace{14mu} 2} \end{matrix}$ It is clear from FIG. 4 that when ΔI_(ON)>ΔI_(OFF) for every consecutive pulse or switching period, the peak current of the next switching period (n+1) will be greater than the peak current in the present switching period (n). An overload condition that reduces the output voltage V_(O) substantially below its regulated value will cause the control circuit to make the off time t_(OFF) as small as possible. Equation 2 shows that reduction of V_(O) with constant or reduced t_(OFF) will reduce ΔI_(OFF). Equation 1 shows that the leading edge blanking time t_(LEB) and the current limit delay time t_(d) impose a minimum value on ΔI_(ON). Therefore, the usual method of control will allow ΔI_(ON) to be greater than ΔI_(OFF) during an overload of the output. When ΔI_(ON) is greater than ΔI_(OFF), the current and voltage can go outside the safe operating area of the switch after only a few consecutive switching periods in which there are short on times that are for example approximately equal in time to the sum of t_(LEB) plus t_(d). Fault detection circuits that respond only after many switching periods are unable to protect the switch from failure. The controller must respond within a small number of switching periods to keep the operation of the switch within its safe operating area. For an embodiment of the present invention, the occurrence of one or more consecutive short on times of the switch, which have an on time approximately equal to the sum of t_(LEB) plus t_(d), may indicate a fault condition.

As will be discussed, a short on time detector may be included for an embodiment of the present invention, which will detect the occurrence of a threshold number of one or more consecutive short on times of power switch S1 120 or 220. For one embodiment, the threshold number may be equal to one. For another embodiment, the threshold number may be equal to two or greater depending on the design. If an occurrence of the threshold number of one or more consecutive short on times in power switch S1 120 or 220 is detected, the next pulse of the drive signal is delayed, which will increase ΔI_(OFF) to reduce the starting value I_(VAL) of the switch current I_(D) through the power switch S1 120 or 220, thereby reducing the chance of the switch current I_(D) from exceeding the current limit in accordance with the teachings of the present invention.

FIG. 5 is a flow diagram that describes a method to keep the operation of the switch within its safe operating area. In general, for one embodiment, the method for regulating a power supply in accordance with the teachings of the present invention includes switching a power switch coupled to an energy transfer element at an input of a power supply in response to a drive signal responsive to feedback received from an output the power supply. If an occurrence of a threshold number of one or more consecutive short on times of the switch is detected, then a next pulse of the drive signal is delayed in response to the occurrence of the threshold number of one or more consecutive short on times of the switch in accordance with the teachings of the present invention.

In particular, for one embodiment, the method measures the on time of the switch to determine whether or not an alternative control action is required to prevent the peak switch current I_(PEAK) from getting too high. For the purposes of the method illustrated in FIG. 5, a short on time is any on time t_(ON) that is not greater than a threshold on time t_(SHORT). For one embodiment, t_(SHORT) is slightly greater than the sum of t_(LEB) plus t_(d). A short on time counter that counts the number of short on times is reset in block 505 before the controller begins the next switching period in block 510. The on time t_(ON) is measured in block 515, and compared to the threshold on time t_(SHORT) in block 520. If the on time t_(ON) is greater than the threshold on time t_(SHORT), the short on time counter is reset in block 505. If the on time t_(ON) is not greater than the threshold on time t_(SHORT), the short on time counter is incremented in block 525. The count of the short on time counter is compared to a maximum number of counts N_(MAX) in block 530. If the number of consecutive short on times is less than N_(MAX), then the next switching period begins after the usual off time for regulation t_(OFF) in block 510. If the number of short on times is not less than N_(MAX), then starting time of the next switching period is delayed in block 535. It is appreciated that at block 535, an occurrence of the threshold number of one or more consecutive short on times is detected. The number of consecutive short on times N_(MAX) can be changed to suit particular applications. For one embodiment the number of consecutive short on times N_(MAX) is fixed permanently for each application. For one embodiment, the number of consecutive short on times N_(MAX) is one. For one embodiment, the number of consecutive short on times N_(MAX) is two. For one embodiment, the number of consecutive short on times N_(MAX) can change dynamically during operation of the power supply in response to conditions or events chosen by the designer.

FIG. 6 shows one embodiment of the method of the present invention in an integrated circuit. Integrated circuit 600 includes power MOSFET switch 628 that is coupled to a drain terminal 602 and a source terminal 646. Switch 628 is coupled to be switched in response to the drive signal 656, which is output from latch 634. A terminal 648 receives a feedback signal U_(FB). For one embodiment, integrated circuit 600 may optionally include a plurality of other terminals to receive signals and to perform functions that are not described in this example.

In the example of FIG. 6, a modulator 626 interprets the feedback signal U_(FB) from terminal 648 to determine whether an enable signal 662 should be high or low. An oscillator 640 provides a clock signal 644 and a maximum on time signal D_(MAX) 664. In the illustrated example, the oscillator 640 includes a FREQ ADJ terminal coupled to an output of latch 638. For one embodiment, the frequency of oscillator 640 changes from a higher frequency to a lower frequency in response to the output of latch 638. Switch 628 responds to the drive signal 656 output from latch 634 that receives signals from AND gate 632 and OR gate 666. Switch 628 is on if both clock 644 and enable 662 are high while both D_(MAX) 644 and OVER CURRENT 658 are low. Switch 628 is off if either D_(MAX) 664 or OVER CURRENT 658 is high.

In the illustrated example, the threshold number of consecutive on times, or, the maximum number of short on times N_(MAX) is two. Accordingly, the latch 638 delays the next switching period after two consecutive short on times by causing oscillator 640 to operate temporarily at a lower frequency in accordance with the teachings of the present invention. The lower frequency has a corresponding longer off time than the higher frequency. A current sensor or limiter is realized with current limit comparator 604, which presents a logic high at a first input of NAND gate 606 when drain current I_(D) 624 exceeds a current limit threshold I_(LIMIT). When switch 628 turns on at the start of a switching period, a second input of NAND gate 606 receives a logic high input after a delay of leading edge blanking time t_(LEB) 622 in response to a high output of drive signal 656 from latch 634. For one embodiment, the leading edge blanking time t_(LEB) is approximately 220 nanoseconds.

As illustrated in the example shown in FIG. 6, a short on time detector is realized in accordance with the teachings of the present invention with NOR gate 612 detecting short pulses from the inputs of inverted OVER CURRENT signal 650, inverted ON signal 654, and delayed ON signal 652. Delayed ON signal 652 is delayed from an active drive signal 656 by leading edge blanking time t_(LEB) 622 and further delayed by a time t_(d1) 610. Delay time t_(d1) 610 is selected to be sufficiently greater than the delay of current limit comparator 604 to guarantee detection of every short on time. For one embodiment, current limit delay time t_(d1) is approximately 100 nanoseconds.

In the example of FIG. 6, AND gate 624 detects long pulses, that are at least as long as the sum of t_(LEB) and t_(d1). Detection of a short pulse by NOR gate 612 sets latch 614. Latch 614 is reset by the detection of a long pulse from AND gate 624.

In the example of FIG. 6, a short on time detector is further realized in accordance with the teachings of the present invention with NOR gate 620 detecting the occurrence of a second short pulse to set latch 638 after two consecutive short pulses. NOR gate 620 receives the same three signals 650, 652, and 654 as NOR gate 612 plus a fourth signal, drive signal 656. The drive signal 656 is represented at the output of latch 614 and delayed by delay t_(d2) 616 and inverted by inverter 618. Delay t_(d2) 616 is sufficiently greater than delay t_(d1) 610 to prevent NOR gate 620 from detecting the first short pulse. For one embodiment, delay t_(d2) 616 is 230 nanoseconds.

In the example of FIG. 6, detection of the second consecutive short on time produces a logic high at the output 660 of NOR gate 620 to set latch 638. A frequency adjuster is realized in accordance with the teachings of the present invention with latch 638 and the FREQ ADJ input of oscillator 640. For one embodiment, when latch 638 is set, oscillator 640 changes from a higher frequency to a lower frequency, which effectively delays the start of the next switching period in accordance with the teachings of the present invention. For one embodiment, oscillator 640 is configured such that setting latch 638 and thereby activating the FREQ ADJ input of oscillator 640 causes the next pulse of the drive signal to be skipped, which effectively causes the frequency of oscillator 640 to be reduced in accordance with the teachings of the present invention. Latch 638 is reset at the start of the next switching period by CLOCK signal 644. If the next pulse is also a short pulse, latch 638 will be set again, reducing the frequency of the oscillator to delay the start of the next switching period. If the next pulse is a long pulse, there will be no delay in the next switching period, and two consecutive short times of switch 628 will be required to set latch 638 in the illustrated example.

In the foregoing detailed description, the methods and apparatuses of the present invention have been described with reference to a specific exemplary embodiment thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive. 

1. A method for regulating a power supply, comprising: switching a switch coupled to an energy transfer element at an input of a power supply in response to a drive signal responsive to feedback received from an output of the power supply; measuring an on time of the switch; determining whether the on time of the switch is a short on time; detecting an occurrence of a threshold number of one or more consecutive short on times of the switch; and delaying a next pulse of the drive signal coupled to be received by the switch in response to the occurrence of the threshold number of one or more consecutive short on times of the switch, wherein delaying the next pulse of the drive signal includes extending an off time of the switch in response to detecting that a switch current exceeds a current limit and if an on time of the switch is less than or equal to a sum of a leading edge blanking period and a current limit delay time period.
 2. The method for regulating the power supply of claim 1 wherein delaying the next pulse of the drive signal in response to the occurrence of the threshold number of one or more consecutive short on times of the switch comprises adjusting a frequency of an oscillating signal from which the drive signal is generated.
 3. The method for regulating the power supply of claim 1 wherein delaying the next pulse of the drive signal in response to the occurrence of the threshold number of one or more consecutive short on times of the switch comprises skipping a next pulse in the drive signal.
 4. The method for regulating the power supply of claim 1 wherein detecting the occurrence of a threshold number of one or more consecutive short on times of the switch comprises detecting a pulse in the drive signal having an on time approximately equal to a sum of a leading edge blanking time and a current limit delay time.
 5. The method for regulating the power supply of claim 1, wherein detecting an occurrence of a threshold number of one or more consecutive on times of the switch includes incrementing a short on time counter for each occurrence of the one or more consecutive short on times.
 6. The method for regulating the power supply of claim 1, wherein extending the off time of the switch includes extending the off time of the switch independent of the feedback signal. 